I know when it comes to stimulant laxatives, you can develop a dependency on them over time and they will eventually cease to work very well to boot. I don't know if people can develop that problem with just fiber, though.
It's not unusual for someone with IBS to have a change in symptoms--for better, for worse, or for about the same as before.
Look your diet over carefully to make sure you haven't added the least newest food, drink, medicine or supplement. Small things can make big changes. Make sure none of your old supplements got a "new and improved" makeover. If you also need exercise to keep regular, make sure you are still meeting your minimum requirements. If you briskly walked on the treadmill before and have replaced it with the same distance, but as a leisurely evening stroll, that might be the difference.
You might also look to see if you have any constipating foods in your diet. Bananas, for one, are constipating (and are even worse in their dehydrated form).